Helicobacter pylori (Hp) chronically infect the human stomach of 50% of the population worldwide. Ten to 20% of those infected will eventually present with severe disease including ulcers and gastric cancers. Our working hypothesis is that Hp disease is a by-product of the interaction between bacterial factors necessary for establishing and maintaining infection and the resultant host defenses. Furthermore, this interaction is dynamic with the bacteria modifying the host and the host modifying the bacteria over decades of infection. To study this complex process, we utilize a mouse model of infection which recapitulates many aspects of human disease including a robust immune response that is unable to clear the infection and alteration of gastric gland architecture. Using this model we will characterize the function of known Hp virulence factors in vivo and perform a saturating screen for new Hp virulence genes. Such a screen has not been possible until recently due to lack of experimental tools. In Aim 1 we examine the in vivo role of the major secreted cytotoxin, VacA, which we were the first to demonstrate has a phenotype during mouse infection. In Aim 2 we describe a screen to identify additional Hp virulence genes. Here we take advantage of two tools we recently developed: an Hp transposon mutant library and a novel methodology we call MATT to monitor transposon mutants in a pool using our Hp cDNA microarray. We believe this screen has the potential to gives us a global view of the Hp genetic requirements for establishing and maintaining infection. Finally, in Aim 3 we investigate the PAI, a group of virulence genes that have been recently shown to mediate a number of specific interactions with cultured cells. We describe experiments that address why no in vivo phenotype for this locus has been described to date and new experiments to measure a role for the PAI during mouse infection. Careful study of infection with mutants in vacA, the PAI genes and newly identified virulence factors have the potential to teach us a great deal about wild type infection by revealing process that fail to occur in mutant infections. These processes likely contribute to the various diseases associated with Hp and may highlight potential therapeutic targets. [unreadable] [unreadable]